A fuel cell is an energy conversion device which electrochemically reacts fuels such as hydrogen and oxygen to produce an electrical current. One particular type of a fuel cell is a Proton Exchange Membrane (PEM) fuel cell. PEM fuel cells have an operating temperature of around 80° C. which makes them favorable for a number of applications, particularly automotive applications.
A PEM fuel cell generally comprises one or more electrically connected membrane electrode assemblies (MEA). Each MEA comprises an anode and a cathode separated by a solid electrolyte allowing for the transfer of protons therethrough. The solid electrolyte is typically in the form of a membrane. The MEAs are disposed between flow fields which provide for distribution of hydrogen across the surface of the anode opposite the membrane and the distribution of oxygen across the surface of the cathode opposite the membrane. To catalyze the reactions at the anode and cathode, catalysts are deposited on the surfaces of the electrodes. A typical catalyst used in PEM fuel cells is platinum.
During operation, hydrogen is supplied to the anode and oxygen is supplied to the cathode to produce an electrical current. The hydrogen and oxygen react at the appropriate electrodes via the following reactions:Anode: 2H2→4H++4e−Cathode: O2+4H++4e−→2H2OOverall: 2H2+O2→2H2O+e−
At the anode, hydrogen is dissociated into hydrogen ions and electrons. The hydrogen ions permeate through the membrane to the cathode, while the electrons flow through an external circuit to the cathode. At the cathode, oxygen reacts with the hydrogen ions and electrons to form water. The flow of electrons from the anode to the cathode via the external circuit may be used as a source of power.
The solid electrolyte as utilized in PEM fuel cells is an acidic proton conducting polymer. The acidity of the polymer allows the transfer of protons from the anode to the cathode while preventing the transfer of electrons therethrough. Sulfonated fluoropolymers are the most popular choice for the acidic proton conducting polymers used in PEM fuel cells. One of the most popular of these conductive polymers is Nafion® (registered trademark of DuPont). The popularity of sulfonated fluoropolymers is due to their high chemical resistivity, ability to be formed into very thin membranes, and high conductivity due to their ability to absorb water. The ability of the sulfonated fluoropolymers to absorb large quantities of water is due to the hydrophilic nature of the sulfonic groups within the polymer. The sulfonic groups provide for the creation of hydrated regions within the polymer, which allow the hydrogen ions to move more freely through the polymer due to a weaker attraction to the sulfonic group. The weaker attraction between the hydrogen ions and the sulfonic groups increases the conductivity of the polymer thereby increasing performance of the fuel cell. As such, the conductivity of the hydrogen ions is directly proportional to the amount of hydration of the sulfonated fluoropolymer.
With the hydration of the electrolyte being an important consideration in PEM fuel cells, the humidity of the air in a PEM fuel cell must be carefully monitored and controlled. If the air has too high of a humidity, the cell can become flooded with water created during operation of the fuel cell resulting in a decrease in performance due to clogging of the electrode pores. If the air has too low of a humidity, the electrolyte may dry out thereby decreasing the conductivity of the electrolyte resulting in decreased fuel cell performance. As such, control systems and humidification systems must be used in conjunction with PEM fuel cells. The use of these systems can adversely affect the cost, size, and mass of PEM fuel cell systems. As such, there is a need in the art for proton conducting polymers which exhibit high conductivity in low humidity environments.